After 5500 trouble free miles on a completely rebuilt 46 knuckle engine (74"). I started to hear a slight noise in the front cylinder… before I got home the noise was louder (piston slap) and I could hear a squeak. Pulling the top end off I found the piston pin bushing had turned in the connecting rod and the front piston and cylinder were scuffed. Everything looked normal in the rear cylinder. I had the front cylinder bored out 0.010” over and installed a new piston, rings, piston pin, and bushing. The rear cylinder was honed and a new set of rings installed. I drove the bike less than 20 miles and started hearing a noise like a leaking head gasket. Before I could pull over the bike started losing power and shut off. I couldn’t turn the engine over with the kicker until it cooled down. The front cylinder was quite cool... the rear was very hot. I pulled the rear cylinder off last night. The head gasket was blown and the rear piston and cylinder are scuffed. I haven’t removed the piston yet, but the pin is squeaking in the bushing. Both times I dismantled the engine, the piston, cylinder, rod, etc seemed quite dry… though this could be from the heat. Does anyone have any idea what might have caused this? Could the turned piston pin bushing in the front cylinder and the blown head gasket in the rear cylinder just be two isolated incidents? Thanks.

Pin was fitted to tight to the bushing and or bushing was not fitted tight enough in the rod. The head gasket may have gone via the excessive hear generated because of the load. Just a guess on my part. Pa

Thanks Paul.The pin fit in the bushings was right... but I don't know about the fit of the bushing in the rod... though the replacement bushing was a standard size and it was tight.... I don't know what to think... maybe an oiling problem... but only to the cylinders?... and after that many miles?...

Pa wrote:Pin was fitted to tight to the bushing and or bushing was not fitted tight enough in the rod. The head gasket may have gone via the excessive hear generated because of the load. Just a guess on my part. Pa

Setting a possible oiling issue aside for the present, or at least until proven, let us look at this from another point of view then. Since we now for a certainty the pin to bushing fit was correct, I submit this, provide oiling is not an issue. Was the bushing oem ? I ask because the material the bushing is made of can make a real difference in expansion amount when heated. If expansion is to much, Instead of the bushing growing a larger diameter [it can't when encircled with the rod bore], the ID of the bushing will shrink. This would explain the bushing spinning in the rod, rather than on the pin.

You bring up a good point Paul... I don't kow if the bushing was oem, or what it was made of, or even if fit right... and that may be something I'll never know now.... I got the rebuilt crank and connecting rods from a friend who is no longer with us after crashing his 40 Knuckle sidecar rig. I don't kow who did the rebuild. Everything was checked and verified except the piston pin bushings. Though, I would have expected the bushing to turn earlier on... not after 5000 miles.

Pa wrote:Setting a possible oiling issue aside for the present, or at least until proven, let us look at this from another point of view then. Since we now for a certainty the pin to bushing fit was correct, I submit this, provide oiling is not an issue. Was the bushing oem ? I ask because the material the bushing is made of can make a real difference in expansion amount when heated. If expansion is to much, Instead of the bushing growing a larger diameter [it can't when encircled with the rod bore], the ID of the bushing will shrink. This would explain the bushing spinning in the rod, rather than on the pin.

JIm Still sounds like a rod oiling problem. Sure wouldn't hurt to make sure a piece of sealer of gasket did'nt migrate in the passage. If this is still the original oiling system it only oils the rod pin for a few degrees every revolution. You could put a few pounds of pressure in the oil passage (use the rocker arm line fitting) and turn the engine over slowly until you see oil coming out between the rods. There should be a corresponding drop in pressure at the same time you see oil coming out between the rods.Just my thoughts, Good LuckF

Thanks F... I agree... it does sound like a rod oiling problem. The bike still has the original oiling system. I want to follow your advice but I'm not quite clear where exactly would I look to see the oil coming out between the rods.

fhsmith1 wrote:JIm Still sounds like a rod oiling problem. Sure wouldn't hurt to make sure a piece of sealer of gasket did'nt migrate in the passage. If this is still the original oiling system it only oils the rod pin for a few degrees every revolution. You could put a few pounds of pressure in the oil passage (use the rocker arm line fitting) and turn the engine over slowly until you see oil coming out between the rods. There should be a corresponding drop in pressure at the same time you see oil coming out between the rods.Just my thoughts, Good LuckF

I'd also bet on an oiling problem, especially if its the original OEM pump. That light goes out at a tiny amount of pressure, and under the best of conditions, you get very little rod bushing oil. After 5500 miles, it is pretty certain that the bushing was right, or close enoungh to right that it didn't just decide to spin on its on unless you could see some sort of clear damage that might have caused that. The scuffing you found when you pulled the rear cylinder again after that 20 miles would further make me suspect oiling problems. The blown head gasket didn't cause the scuffing, but the scuffing and the very hot rear cylinder might help cause the head gasket to blow. Of course, its just as good a guess that the head gasket might have blown simply because you didn't get it torqued just right. As you know, knuck head gaskets can be tricky to get right. I like to use the pure copper head gaskets from Cometic just because I couldn't get any of the others to stay sealed on my current '40 engine. I had a very similiar problem on a '47 knuck I rode daily back in the late 60's and early 70's. I kept having pistons collapse. C.B. Clausen at M.C. Suppy in L.A. was where I was getting my pistons and they were doing the machine work fitting them and it didn't make sense to them either. C.B. finally told me that it had to be the oiling system and found that the oiling system wasn't supplying enough oil to the inside of the cylinders. He was able to clean up the passages and that was the end of the problem. It did take pulling the cases apart though.

If the engine has enough oil pressure to turn the light off, that only proves it has what? 2 LBS? (I don't recall the exact spec, but it is not much) That might be enough oil pressure for idle, but that about all. If it was mine, I would be pulling the oil pump for inspection. If for no other reason than for peace of mind and to eliminate the most obvious possibility.

That engine was getting nearly no oil through the pinion and as a result, nearly no splash. The cylinder walls and piston skirts would get way over heated, and the pistons would collapse and start slapping. This would happen within about 25 miles of my doing a top end. I don't even remember why the passage was either restricted or blocked. It was a long time ago. But that was what C.B. told me he found and after he cleaned it up and put the bottom end back together - I didn't know how to do a bottom end back then - I never had another piston collapse for many more years until I sold the bike for all of about $600. I still shake my head over that one. I think C.B. charged me all of about $100 for the job too. He was the guy who built and owned "The Brute" back then. Dual engine UL Flathead he ran at Bonneville though he didn't ride it. It was sort of famous in the 50's and very early 60's.

Between fork and knife (front and rear) rod. Also between the fork rod and flywheels. This is looking from where the cylinders were removed. Be careful, the oil will only flow for a very few degrees of engine rotation. Turn the engine very slowly with pressure applied. You will see pressure drop when the oil hole in the pinion aligns with the hole in the bushing. That's when you should see the oil.F